Field of the Invention
The present invention relates to an automatic transmission for a vehicle. More particularly, the present invention relates to an automatic transmission for a vehicle that improves power delivery performance and fuel consumption and obtains a linearity between step ratios of transmission steps by enlarging a span of gear ratios while achieving nine forward speed stages using a minimum number of constituent elements.
Description of Related Art
Recent increases in oil prices are triggering stiff competition in enhancing fuel consumption of a vehicle.
Accordingly, research is being conducted on engines in terms of reducing weight and improving fuel efficiency by down-sizing, and research is also being conducted to ensure both drivability and competitiveness by maximizing fuel efficiency by implementing an automatic transmission with multiple speed-stages.
However, in order to achieve more shift stages for an automatic transmission, the number of parts is typically increased, which may deteriorate installability and/or power flow efficiency and may increase production cost, and weight.
Therefore, in order to increase an effect of improving fuel efficiency by implementing an automatic transmission with multiple speed-stages, it is important to develop a planetary gear train capable of maximizing efficiency using a small number of components.
In this respect, recently, an eight-speed automatic transmission has been implemented, and research and developments is being actively conducted on a planetary gear train that may implement gear shift stages for eight or more speeds.
However, in the case of the recent eight-speed automatic transmission, a span of a gear shift ratio is maintained at a level of 6.5 to 7.5, and as a result, there is a problem in that the recent eight-speed automatic transmission has no great effect of improving fuel efficiency.
In a case in which a span of a gear shift ratio in the eight-speed automatic transmission is increased to the level of 9.0 or more, because it is impossible to ensure linearity of step ratios between gear shift stages, driving efficiency of the engine and drivability of the vehicle deteriorate.
Accordingly, there is a need for development of a highly efficient automatic transmission with 9 or more forward speed stages.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.